The present invention relates to an electrically conductive feedthrough connection through a hole of a high-temperature-resistant and vacuum-proof insulating part, particularly of ceramic, glass, or a single crystal, which feedthrough connection is designed as a terminal lead covered with active solder and soldered into the hole. The terminal lead has a coefficient of thermal expansion less than that of the insulating part. The connection is used in a pressure sensor comprising a diaphragm and a substrate which have spaced-apart, flat inner surfaces, provided with at least one conductive or resistive layer for forming at least one capacitor or strain gage, respectively, and wherein the conductive surfaces are electrically connected to the respective rear side via the feedthrough connection. The connection is manufactured by inserting the covered lead into the hole and then the insulating part and inserted lead are placed in a vacuum, or a gas atmosphere with a pressure not exceeding 10 mbars, and then heated until the active solder has completely melted.
The electrically conductive feedthrough connection through a hole of a high-temperature-resistant and vacuum-proof insulating part, particularly of ceramic, glass, or a single crystal, is designed as a terminal lead covered with active solder and inserted into the hole, the terminal lead having a coefficient of thermal expansion less than that of the insulating part.
The feedthrough connection is used in a pressure sensor comprising a diaphragm and a substrate which have spaced-apart, flat inner surfaces which are provided with at least one conductive layer or resistive layer for forming at least one capacitor or strain gage, respectively, and are electrically connected to the respective rear side via the feed-through connection.
For the manufacture of a feed-through connection, the covered lead is inserted into the hole, and the thus equipped insulating part is placed in a vacuum and heated until the active solder has completely melted.
For the manufacture of a feed-through connection, the covered lead is inserted into the hole, and the thus equipped insulating part is heated in a gas atmosphere with a pressure not higher than 10 mbars (=1 kPa).
According to the journal "Solid State Technology", April 1985, pages 321 to 324, the commonly employed method of manufacturing such feed-through connections in alumina ceramic involves the use of an Mn--Mo paste which must be applied to the wall of the hole, sintered at a high temperature (approximately 1500.degree. C.) in moist hydrogen, and subsequently electroplated with nickel. The nickel layer must be sintered as well. In the hole thus metallized, a terminal lead can then be soldered into place.
Because of the number of process steps to be performed, this manufacturing method is very complicated and costly. In addition, process control is highly critical, e.g., because of the danger of explosion resulting from the use of moist hydrogen, which requires expensive safety precautions or a special furnace. Furthermore, in the case of long, thin holes, the Mn--Mo paste can be applied only by hand, the applied layer having to be of uniform thickness. Moreover, Mn--Mo paste is not particularly suitable for very-high-purity alumina ceramics.
Accordingly, the object of the invention is to provide a feed-through connection which is generally suitable for high-temperature-resistant and vacuum-proof insulating parts, particularly of ceramic, glass, or a single crystal, and not only for alumina-ceramic parts, can be manufactured in a single high-temperature step and is inexpensive, mechanically heavily loadable, and high-vacuum-tight.
The active solder used in the invention consists of a soldering material, mostly a brazing solder such as Ag, Ag--Cu, or Ag--Cu13 In, alloyed with at least one reactive element, such as Ti, Zr, Be, Hf, or Ta, with Ti having proved to be the most effective alloying element. During the soldering process, the reactive element wets the surfaces of the parts to be soldered, so that no metallization, such as the above-mentioned Mn--Mo coating, is necessary.
In the case of oxide ceramic, the high affinity of the reactive element for oxygen causes a reaction with the ceramic, which leads to the formation of mixed oxides and free valency electrons. Active solder can also be used with nonoxide ceramic or glass without previous metallization.
Preferred active-solder alloys are ductile and contain 2 to 5% of Ti which is homogeneously embedded in a matrix of, e.g., Ag--Cu. They can be processed like normal brazing solders, so that the terminal leads, too, can readily be covered with them.
Typically commercially available active solders are the alloys Ag--Ti, Ag--Cu--Ti, and Ag--Cu--In--Ti, whose soldering temperatures range between 750.degree. and 1000.degree. C. Thus, step soldering (gradations in the melting points) is also possible with active solders. The strengths of active solders are identical with the strengths of comparable Ti-free brazing solders. The bond strength to ceramic, for example, is greater than, the strength of the ceramic itself; in a tensile test, the fracture will therefore lie in the ceramic, not in the ceramic-to-solder interface.
The active solder is preferably heated in a vacuum at at least 10.sup.-5 mbars (=10.sup.-3 Pa), better in the 10.sup.-6 mbar (=10.sup.-4 -Pa) range. Very good vacuum is necessary in order to avoid reactions of the Ti with the residual gas and achieve good wetting of, e.g., ceramic.
To obtain specific soldering results, e.g., to reduce the evaporation of the solder or to reduce surface oxides, it may be advantageous to carry out the heating or soldering process in a defined gas atmosphere of inert gas and/or reactive gas. The partial pressures of these gases are preferably below 10 mbars (=1 kPa).
During active soldering, like during conventional soldering, the solder is completely melted. The soldering temperature of the active solder, however, should be 70.degree. to 100.degree. C. above the liquidus temperature to obtain an optimum reaction of the Ti with, e.g., ceramic. In this manner, high strength and vacuum tightness are achieved.
Pressure sensors with electrical feed-through connections in accordance with the invention are characterized by high mechanical strength, loadability, and resistance to temperature changes as well as by very good and very reliable vacuum tightness while being easy to manufacture. In addition, the quality of the feed-through connection can be examined quickly and simply by radiography with X-rays.
It is surprising that despite the very different temperature dependencies of the expansion coefficients of metal and, e.g., ceramic, an active solder can be used for soldering in feed-through connections, particularly of pressure sensors.
Further features and advantages of the invention will become apparent from the following description of an embodiment, which is illustrated in the accompanying drawings.